The purpose of this work is to identify relationship between fetal growth, availability of nutrients essential for fetal growth and patterns of hormone metabolism by mother and fetus. Evidence suggests that the fetus may regulate maternal physiology and control the availability of substrates through synthesis and release of steroid hormones. We have detected systematic relationships between steroid hormones and (a) rate of uterine blood flow; (b) O2 consumption by the fetus, placenta and uterus; (c) maternal arterial O2 content and birthweight in normal, near term sheep. In present work we will test the significance of these relationships by exposing sheep to environmental conditions (heat stress) which are known to diminish birth weight. We then will measure which variables undergo commensurate change; which relationships are altered.